Electron microscope with energy analyzer

ABSTRACT

An electron microscope in which the image forming lens system is employed as an energy analyzer. During beam energy analysis, an electron beam which passes through a slit positioned along the optical axis adjacent an objective lens is deflected so as to strike an intermediate lens off-axially by a deflecting means provided between said slit and said intermediate lens whereby the beam is dispersed by field (off-axis) chromatic aberration of the intermediate lens.

United States Patent Shirota ELECTRON MICROSCOPE WITH ENERGY ANALYZER Inventor: Kohei Shirota. Tokyo, Japan Nihon Denshi Kabushiki Kaisha, Tokyo, Japan Filed: Jan. 23, 1974 Appl. No.: 435,725

[73] Assignee:

[30] Foreign Application Priority Data Jan. 31. 1973 Japan 4842654 US. Cl. 250/305, 250/3l 1 Int. Cl. l-l0lj 37/26 Field of Search 250/31l, 3 l0, 307, 306,

References Cited UNITED STATES PATENTS 6/1966 Watanabe 250/3ll 1 Jan. 28, 1975 3,6l9,607 ll/l97l lchinokawa 250/311 Primary Examiner-Archie R. Borchelt Assistant E.ruminer-C. E. Church Attorney, Agent, or Firm-Webb, Burden, Robinson & Webb l 57 ABSTRACT 5 Claims, 9 Drawing Figures Pmmngmzems 3,863,069

SHEET 1 BF 3 DEFLECTINQ- POWER SOURCE.

DEFLECTlNL- Pam/ER SOvR E PMENTEDMNZBIQYS 3,863,069

SHEET 2 UP 3 PRIOR ART izg. 5

ELECTRON MICROSCOPE WITH ENERGY ANALYZER This invention relates to an apparatus for measuring the energy of an electron beam by utilizing the imageforming lens system in a conventional electron microscope.

In a conventional transmission electron microscope, a microscopic image is formed by passing an electron beam through a thin specimen. In this case, from the point of view of solid state physics, it is of prime importance to analyze the specimen by measuring the energy of the electron beam passing through a specific spot on the specimen. In order to do this, however, it is necessary to install an electron beam energy analyzer which not only functionally restricts the microscope but also adversely affects the operability of the instrument.

A proposition was recently put forward to the effect that the various aberrations inherent in the microscope lenses be utilized as an energy analyzer. This approach, which utilizes the fact that the field (off-axis) chromatic aberrations of the other (objective, projector) lenses making up the image forming lens system, involves the utilization of an aperture which is arranged between the objective and intermediate lens so as to make only the electron beam which passes through a microarea of the specimen at some distance from the optical axis irradiate the intermediate lens off-axially.

By so doing, the electron beam path is split by the intermediate lens field chromatic aberration in accordance with the energy of the electron beam. As a result, said aperture image is also split, an energy spectrum is formed and magnified images are projected onto a fluorescent screen or photoplate by the projector lens.

In the above described energy measuring method, however, although eliminating the necessity of a conventional energy analyzer, it does have a very decided disadvantage in that, since the electron beam is passed through the objective lens, off-axially, the resolution of the energy deteriorates due to the effect of the field chromatic aberration of said lenses. Further deterioration of said energy resolution can also be attributed to the aberrations of the other lenses.

An advantage of this invention is to provide an electron microscope free from the effects of objective lens field chromatic aberration when using the intermediate lens as an energy analyzer. Another advantage of this invention is to provide an intermediate lens whose aberration blurs other than the field chromatic aberration blur are small enough for analyzing low energies.

Briefly, according to this invention, a typical transmission electron microscope is provided with additional features which enable the use of the field (offaxial) chromatic aberration of the intermediate lens to disperse and thereby enable analysis of the energies of the electrons in the electron beam. In a transmission electron microscope, according to this invention, an electron gun creates an electron beam which is condensed and directed by a condensing lens system. Preferably, the condensing lens system comprises two condenser lenses enabling the direction of the electron beam along an optical axis for striking the thin specimen substantially perpendicular thereto. The microscope includes an image forming lens system comprising at least an objective lens and an intermediate lens and usually a projector lens. The specimen is placed near or within the objective lens in a typical fashion. A

shiftable baffle is provided with a slit therein to be positioned (during beam energy analysis) aligned with the optical axis in the image forming lens system to substantially prevent an electron beam passing through said specimen off the axis from also passing through said slit. This is achieved, for example, by placing the slit at the image plane of the objective lens. Deflecting devices such as deflecting coils (during the beam energy analysis) deflect the electron beam passing through said baffle to enter the intermediate lens offaxially so that the intermediate lens disperses the beam according to the energies of the electrons therein. The dispersed electron beam is then directed at an imaging device such as a fluorescent screen. A projector lens may project the dispersed beam upon the screen. Preferably, a deflection coil deflects the dispersed beam to pass through the vicinity of the front focal point of the projector lens so that the image of the dispersed beam will be about at the center of the screen placed at the imaging plane of the projector lens. Of course, the deflection devices are disabled when the microscope is being used for creating transmission electron images and not for analyzing the energies of the transmitted electron beam. It may also be necessary to withdraw the baffle from the electron optical axis.

These and other advantages of this invention will become apparent by reading the following description in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic diagram showing the electron microscope according to this invention;

FIG. 2 is a schematic diagram showing the electron beam path of the microscope optical system shown in FIG. 1;

FIGS. 3 and 4 are schematic diagrams showing the electron beam paths of the embodiments according to this invention respectively;

FIG. 5 is a schematic diagram of an optical system in which the intermediate lens of a conventional apparatus is used as an energy analyzer;

FIG. 6 is a schematic diagram for explaining the field chromatic aberration of an objective lens;

FIG. 7 is a schematic diagram for explaining the field chromatic aberration of an intermediate lens;

FIG. 8 is a schematic diagram for explaining the various aberrations of an intermediate lens; and,

FIG. 9 is a schematic drawing of magnetic pole pieces of the intermediate lens.

FIG. 1 is a sectional view of a microscope column 1 at the upper end of which an electron gun 2 comprising a filament, a Wehnelt electrode 3, and an anode 4 is provided. Condenser lenses 5 and 6 operate to make the electron beam generated by electron gun I run almost in parallel with the optical axis thereby striking a specimen 7 more or less perpendicularly. The specimen 7 is positioned in the gap between the objective lens magnetic pole pieces by means of a specimen holding and manipulating device (not shown) installed in the specimen chamber 8. The electron beam, thus irradiating the specimen 7, passes through an image forming system consisting of an objective lens 9, an intermediate lens 10 and a projector lens 11 which operates to form and project a magnified image of the specimen on a fluorescent screen 12 installed in a viewing chamber 13 equipped with a viewing window 14.

The embodiments according to this invention are provided with a shifting device 15, located between the objective lens 9 and the intermediate lens 10, for inserting and withdrawing a baffle having a slit 16 therein to and away from the optical axis. The stages of deflection coils 17a and 17b, complete with a power source 18 are located between the objective lens 9 and the intermediate lens 10. A single deflection coil 19, complete with a power source 20 is located between the intermediate lens and the projector lens 11.

FIG. 2 is a ray diagram showing the center axis of the electron beam paths obtained by using the apparatus illustrated in FIG. 1. Of the electron beams passing along the optical axis 21 of the electron optical system, those having the same energy, after passing through the slit 16 are deflected by deflection coils 17a and 17b and enter the intermediate lens 10 off-axially.

By manipulating the slit shifting device (not shown) the slit can be located close to the axis 21 on the image plane of the objective lens 9 and by controlling the current passing through the deflection coils 17a and 17b, the electron beam can be deflected away from the slit. The electron beam having entered and passed through the intermediate lens off-axially forms a slit image at the imaging position of the intermediate lens which is projected onto the fluorescent screen 12 by the projector lens 11. In this case, without the aid of the deflection coil 19, the electron beam would enter the projector lens off-axially and the slit image would appear well off-center of the fluorescent screen as shown by the broken line. With the aid of said coil, however, the electron beam is deflected to pass through the projector lens in the vicinity of the lens axis thereby projecting a magnified slit image onto the approximate center of the fluorescent screen as shown by the solid line. As a consequence, image observation without the adverse effects attributable to projector lens field aberration is made possible.

FIG. 3 is a ray diagram showing the electron beam path obtained by a simplified version of the apparatus illustrated in FIG. 1. In this arrangement the deflection coil I9 has been dispensed with and one stage deflecting coil 17c located just behind the slit I6 is used instead of the two deflecting coils 17a and 17b. As a result, a simple optical system is provided which is almost as effective as the one shown in FIG. 2.

FIG. 4 is a ray diagram showing the center axis and electron beam paths in one embodiment according to this invention, which explains the principle of this invention. Here, an electron beam having two different energies is split into two beam components by the field chromatic aberration of the intermediate lens with the result that each respective beam component produces a slit image on the fluorescent screen 12. Now, when an electron beam having a constant energy irradiates a specimen, information on the specimen can be obtained by analyzing the energy of the transmitted electrons since part of the energy, depending on the specimen material, is lost when the beam passes through the specimen. Accordingly, in the optical system illustrated in FIG. 4, many slit images, i.e. energy spectra, are produced on the fluorescent screen 12.

By way of comparison, FIG. 5 is a ray diagram showing an optical system not according to this invention, in which the intermediate lens of a conventional apparatus is used as an energy analyzer. In this optical system, the slit 16 is located at some distance from the optical axis so that the electron beam enters the intermediate lens off-axially. In such an optical system, the electron beam which passes through a slit 16 is affected by the field chromatic aberration of the objective lens because said beam has passed through the objective lens offaxially.

FIG. 6 illustrates the field chromatic aberration of an objective lens. As shown in the figure, the two electron beams passing through the lens off-axially have focal lengths which differ according to the energy of the respective electron beams. As a result, it is impossible to determine, with any degree of accuaracy, the point or microarea of the specimen through which the electron beam passing through the slit arranged behind the objective lens passes. For example, if the energy loss of the electron beam passing through the specimen at point A is greater than that of the electron beam passing through the specimen at point B, a situation is created whereby the possibility of both beams passing through the slit, positioned along the image forming plane, at the same time cannot be eliminated. Accordingly, in the measuring method according to the prior art, the energy spectrum is measured without knowing for sure through which part of the specimen the electron beam is passed. On the other hand, in the optical system according to this invention, the adverse effect of the objective lens field chromatic aberration is eliminated by utilizing the electron beam passing through the center of the objective lens.

FIG. 7 is a ray diagram for explaining the field chromatic aberration of an intermediate lens. The diagram shows the electron beam path for dispersing the energy of a spot image formed by an objective lens at a distance Z, from an intermediate lens, the image of said spot image being formed on the image forming plane of said intermediate lens located at a distance Z, from said intermediate lens on the projector lens (not shown) side. The energy dispersion 8H on the image plane is expressible as where a is the incident angle of the electron beam. M, is the magnification of the lens, V is the accelerating voltage of theelectron beam, AV is the variation width of the electron beam energy, and Cr is the (onaxis) chromatic aberration coefficient of the intermediate lens. Now, if we let the spherical aberration coefficient of the intermediate lens equal C and the divergent angle of the electron beam equal y, the various lens aberrations for example in F IG. 8. the amount of de-focus A fi, image blurring SSL and image shift 1-1 in terms of the image forming plane of the intermediate lens can be expressed as follows:

Afi=3'C,,-01 M,

5si=3'C,,a "yM,

ri=C,,'a 'M Thus, by simple deduction, the above equation (I), (2), (3) and (4) indicate that when using an intermediate lens as an energy analyzer, the field chromatic aberration coefficient C should preferably be as large as possible and the spherical aberration coefficient C, should preferably be as small as possible.

Now, the half-width value of the magnetic field distrito a microscope using a three-stage or more image bution of the magnetic field produced by the intermediforming lens system comprising, for example, an objecate lens magnetic pole pieces shown in FIG. 9' is extive lens, two intermediate lens, and two projector lens. pressed as Having thus described the invention with the detail 5 and particularly as required by the patent laws, what is D z (H-b) desired protected by Letters Patent is set forth in the (5 following claims.

I claim: b z +b2/2 l. A transmission electron microscope comprising:

' (6) a. means for creating, condensing and directing an electron beam to irradiate a transmission speciwhere, s is the distance between an upper pole piece 23 and a lower P Piece 24 and 1, 2 e the inside b. an image forming lens system comprising at least diameters of the pp P Pieces and the lower an objective lens and intermediate lenses defining pole pieces as shown in FIG. 9. an i l axis,

On the assumption that the lens magneticfield pro- 0. shiftable baffle means having a slit therein posiduced by such pole pieces is Bell-shaped as manifested tionable during beam energy analysis to align the by the theory based on Glasers Aberration Theory; slit with the optical axis of th image forming lens to moreover, by taking into account the fact that the absosubstantially prevent an electron beam passing lute value |Z of the distance Z 0) between the through said specimen off the axis from passing object plane and the lens principal plane and the disthrough said slit, tance Z, 0) between the image plane and said prind. meansfor deflecting the electron beam passed cipal plane are sufficiently larger than the half-width through said baffle means during beam energy value D, C and C, can be expressed approximately as a 7 ana y Causing it 9 enter the intermediate l f ll 25 off-axially such that the intermediate 16 ns disperses the beam according to the energy of the electrons r m) '1 0) o/ 1) therein, and I I (Z1 e. imaging means upon which the dispersed beam is (7) projected such that the beam energy may be analyzed. k2m l 2 An electron microscope as set forth in claim 1 in C W 4 4k2m+ 3 which said deflecting means incorporated two stages of deflecting coils arranged between said baffle and said intermediate lens.

. l' L 3. An electron microscope as set forth iiTclaim l in /k m l /k m l which deflecting means comprises one stage of deflecting coils located adjacent said baffle. D Z8 Z0 4. An electron microscope as set forth inc laim 1 in m (a which said intermediate lens sat sfies the relation s +1 40 b/Z 2 50 mm where s is the distance between upper I pole pieces and lower pole pieces and b and b are the where km is a parameter indicating the intensity of thhe I inside diameter of the pp lower Pewer PleeeS lens and til is the angle obtained by normalizing the disrespective]? j tance Z and 2, with. the half-width value D of the lens A transmission electron 'P P p 'f magnetic field. From equations (7) and (8), it is clear means for Creating, coljdensmg and f lf that although the chromatic aberration C is unrelated electron beam "Tadlate a transmlsslo" p to the half-width value D, the spherical aberration coefficient C, is in inverse proportion to the square of said 1 8? formmg System p g a! least half-width value D indicating that the half-width value an ls l and mtermedlate lenses definmg D of an intermediate lens used as an energy analyzer Pp 3X15, h ld b as large as p ibl c. shiftable baffle means having a slit therein post- In most commonly used electron microscopes having P P durmg beam'energy' analysis align the accelerating voltages f about 100 1,000 Kv the sl1t w|th the optical axis of th image forming lens to half-width value of the intermediate lens pole pieces is ally prevent an electron beam pass ng normally less than 10 mm. Accordingly, with such mithrough 531d speclmen off the from passmg croscopes, if the image forming lens system is used as d j n th I t b d an energy analyzer, it would be impossible to obtain r i 2 n e gjs zi 5:2 sufficient energy resolution. mug Sal a e ea 8 g gy A feature of this invention is that the half-width value analysis causmg It W i i fi lens of the intermediate lens magnetic pole pieces is 13 mm off'axlany Such that t e mterme late ens lsperses or more; that is to say, s b is larger than 50 mm. An intermediate lens satisfying such requirements is capable of reducing lens aberrations other than field chromatic aberration. to a minimum; in addition to which, the image-forminglens system, when used as an electron energy analyzer would guarantee sufficient en- ..is notres r cted. t6 the abo e through the following stage lenses in the vicinity of the lens axis. described embodiments; for example, it can be applied therein,

e. imaging means upon whichthe dispersed beam is projected such that the beam energy may be analyzed, and

f. second means for deflecting the electron beam passed through said intermediate lens so as to pass the beam according to the energy of the electrons 

1. A transmission electron microscope comprising: a. means for creating, condensing and directing an electron beam to irradiate a transmission specimen, b. an image forming lens system comprising at least an objective lens and intermediate lenses defining an optical axis, c. shiftable baffle means having a slit therein positionable during beam energy analysis to align the slit with the optical axis of th image forming lens to substantially prevent an electron beam passing through said specimen off the axis from passing through said slit, d. means for deflecting the electron beam passed through said baffle means during beam energy analysis causing it to enter the intermediate lens off-axially such that the intermediate lens disperses the beam according to the energy of the electrons therein, and e. imaging means upon which the dispersed beam is projected such that the beam energy may be analyzed.
 2. An electron microscope as set forth in claim 1 in which said deflecting means incorporated two stages of deflecting coils arranged between said baffle and said intermediate lens.
 3. An electron microscope as set forth in claim 1 in which deflecting means comprises one stage of deflecting coils located adjacent said baffle.
 4. An electron microscope as set forth in claim 1 in which said intermediate lens satisfies the relation s + b1 + b2/2 > or = 50 mm where s is the distance between upper pole pieces and lower pole pieces and b1 and b2 are the inside diameter of the upper and lower power pieces respectively.
 5. A transmission electron microscope comprising: a. means for creating, condensing and directing an electron beam to irradiate a transmission specimen, b. an image forming lens system comprising at least an objective lens and intermediate lenses defining an optical axis, c. shiftable baffle means having a slit therein positionable duRing beam energy analysis to align the slit with the optical axis of th image forming lens to substantially prevent an electron beam passing through said specimen off the axis from passing through said slit, d. first means for deflecting the electron beam passed through said baffle means during beam energy analysis causing it to enter the intermediate lens off-axially such that the intermediate lens disperses the beam according to the energy of the electrons therein, e. imaging means upon which the dispersed beam is projected such that the beam energy may be analyzed, and f. second means for deflecting the electron beam passed through said intermediate lens so as to pass through the following stage lenses in the vicinity of the lens axis. 